The present invention relates to an optical element for an optical pickup device, aberration correcting element, light converging element, objective optical system, optical pickup device using these optical elements, and optical information recording reproducing device.
Recently, in the optical pickup device, a tendency to reduce the wavelength of a laser light source used as the light source for recording the information into an optical disk, is advanced, for example, a laser light source of a wavelength 405 nm such as a blue violet semiconductor laser or a blue violet SHG laser in which the wavelength conversion of the infrared semiconductor laser is conducted by using the second harmonics generation, is being put to practical use.
When these blue violet laser light sources are used, in the case where an objective lens whose numerical aperture (NA) is the same as that of DVD (digital versatile disk) is used, the information of 15-20 GB can be recorded in an optical disk of 12 cm diameter, and in the case where NA of the objective lens is increased to 0.85, the information of 23-25 GB can be recorded in the optical disk of 12 cm diameter. Hereinafter, in the present specification, the optical disk and optical magnetic disk using the blue violet laser light source are generally called as “high density optical disk”.
Hereupon, in the high density optical disk using the objective lens of NA 0.85, because a coma generated caused by the skew of the optical disk is increased, a protective layer is designed thinner than in DVD (0.1 mm to 0.6 mm of DVD), and an amount of the coma due to the skew is reduced.
Hereupon, only by a fact that the information can be appropriately recorded/reproduced for such a high density optical disk, it can not be said that a value as a product of an optical disk player is sufficient. In the present time, when the actuality that DVD and CD (compact disk) in which various kinds of information are recorded, are sold in a market, is taken into account, it is not sufficient only by a fact that the information can be recorded/reproduced for the high density optical disk, but, for example, when it is structured in such a manner that, also to DVD or CD which is owned by the user, the information can be appropriately recorded/reproduced in the same manner, it leads to an increase of the product value as an optical disk player for a high density optical disk. From such a background, in the optical pickup device mounted on the optical disk player for the high density optical disk, it is desired that it has the performance by which the information can be appropriately recorded/reproduced, while keeping the interchangeability also for any one of the high density optical disk and DVD, further, CD.
A method is considered by which the optical parts for the high density optical disk and the optical system for DVD or CD are selectively switched corresponding to the recording density of the optical disk for which the information is recorded/reproduced, as a method by which the information is appropriately recorded/reproduced while keeping the interchangeability also for any one of the high density optical disk and DVD, further, CD, however, because a plurality of optical systems are necessary, it is disadvantageous for size-reduction, further, the cost is increased.
Accordingly, for the purpose of simplification of the structure of the optical pickup device and cost-reduction, also in the optical pickup device having the interchangeability, it is preferable that the optical system for the high density optical disk and the optical system for DVD or CD are in common with each other, and the number of optical parts constituting the optical pickup device is reduced as largely as possible.
As the objective optical system for the optical system which can be used in common with a plurality of kinds of optical disks whose recording density is different each other, a technology by which the ring shaped zone structure around the optical axis as written in Patent Documents 1 and 2, is provided on the lens surface, and in respective ring shaped zones, a plurality of concave and convex structures are formed, is well known.
(Patent Document 1) Tokkaihei 9-306018
(Patent Document 2) Tokkai 2002-277732
The technology written in 2 above-described Patent Documents is a technology in which, when the depth of a stepped section of the concave-convex structure formed in the ring shaped zone is made a depth so that practically the phase difference is not added in mutual adjoining concave-convex structures to the wavelength (for example, λ1) of either one of the recording/reproducing wavelength λ1 of DVD or the recording/reproducing wavelength λ2 of CD, it is made in such a manner that, by the concave-convex structure, the phase difference is given only to the other side wavelength (for example, λ2).
Further, by the number of concave-convex structures formed in each of ring shaped zones, because when the light flux of wavelength λ2 passes the ring shaped zone structure, the phase difference of integer times of the wavelength is given in mutual adjoining ring shaped zones, only the light flux of wavelength λ2 is diffracted by the ring-shaped zone structure. The concave-convex structure formed in each ring shaped zone in this case, is set so that both transmission factors (diffraction efficiency) for the wavelength λ1 and the wavelength λ2 are largely secured.
In the objective optical system written in Patent Document 1, when the light flux of the wavelength λ2 is diffracted by the ring-shaped zone structure, the light flux of the wavelength λ2 is projected as the diverging light flux so that spherical aberrations generated due to the difference of protective layer thickness between DVD and CD are cancelled, and in the objective optical system in Patent Document 2, when the light flux of the wavelength λ2 is diffracted by the ring shaped zone structure, because the spherical aberration by which spherical aberrations generated due to the difference of protective layer thickness between DVD and CD are cancelled, is added to the light flux of the wavelength λ2, recording/reproducing of the information for the DVD and CD can be conducted by a common objective optical system.
Both of technologies disclosed in Patent Documents 1 and 2 are technologies by which the interchange between 2 kinds of optical disks of DVD and CD is realized, and because there is no disclosure for the optimum ring shaped zone structure (for example, the number of concave-convex structures formed in each ring shaped zone) for the purpose in which, for the recording/reproducing wavelength (the vicinity of 400 nm) of the high density optical disk and the recording/reproducing wavelength (the vicinity of 650 nm) of DVD, the spherical aberrations generated due to the difference of the protective layer thickness between the high density optical disk and DVD are corrected, and the high transmission factor (diffraction efficiency) is secured, for the purpose in which the interchange between the high density optical disk and DVD is realized, it is difficult that the technologies disclosed in the above-described Patent Documents are applied as they are.
Further, in order to conduct the recording/reproducing of the information by using the common objective optical system on the high density optical disk and DVD, as described above, other than the spherical aberration generated due to the difference of the protective layer thickness between the high density optical disk and DVD, it is necessary that problems proper to the high density optical disk are solved.
Problems proper to the high density optical disk are (1) the chromatic aberration accompanied by a reduction of wavelength of the laser optical source (2) the spherical aberration change accompanied by an increase of numerical aperture. In them, (1) is the problem actualized for the reason that the wavelength dispersion (a change of refractive index to a minute wavelength change) of optical materials is large in the blue violet wavelength area. When the mode is switched for the optical disk, from the reproducing of the information to the recording or from the recording of the information to the reproducing, because the output of the semiconductor laser light source is changed, the oscillation wavelength is changed (so-called mode-hopping). This wavelength change is about several nm, however, because the wavelength dispersion is large in the blue violet wavelength area, while the objective optical system is focused again, it is in the de-focus condition, and the adequate recording/reproducing characteristic is not obtained.
Further, (2) is a problem actualized for the reason that the spherical aberration generated in the objective optical system is increased in proportion to fourth power of the numerical aperture. In the objective optical system of high numerical aperture, because the spherical aberration when the wavelength of the incident light flux is changed, is increased, the allowance for the wavelength of the laser light source becomes severe. Particularly, because there is an influence of the wavelength dispersion in the blue violet wavelength area, this problem is more actualized. Further, in order to reduce a production cost, it is effective that the objective optical system is formed of a plastic lens, however, because the spherical aberration generated due to the refractive index change accompanied by the temperature change is increased, when the temperature in the optical pickup device is changed, the recording/reproducing characteristic of the information for the high density optical disk is interfered.